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 SUMMARY Now that you have completed this chapter, let's review some of the new terms, concepts, and ideas you have learned. You should have a thorough understanding of these principles before moving on to chapter 2. WAVE PROPAGATION is an invisible force that enables man to communicate over long distances. Wave transmission can take many forms, such as LIGHT, SOUND , and RADIO. LIGHT is a form of wave motion that can be seen. Heat cannot normally be seen, but can be felt. Radio waves cannot be seen or felt. WAVE MOTION can be seen in action by throwing a pebble into a pool of still water. The ripples that move toward the edge of the pool demonstrate the PROPAGATION theory. The TRANSVERSE WAVE is a type of wave motion. Radio, light, and heat waves are examples of transverse waves. The LONGITUDINAL WAVE is another type of wave motion. The sound wave is the only example of a longitudinal wave given in this text. SOURCE, MEDIUM, AND DETECTOR (RECEIVER) are the three requirements for all wave motion. A SOURCE can be anything that emits or expends energy (waves). The MEDIUM is the vehicle for carrying waves from one point to another. Water, air, metal, empty space, etc., are examples of a medium. Empty space is considered a medium for electro- magnetic waves but not a medium for sound waves. The SOUND DETECTOR absorbs the waves emitted by the source. The human ear is an example of a detector. HERTZ, which is abbreviated Hz, is used in lieu of "cycle per second" when referring to radio frequencies. VELOCITY OF PROPAGATION is the speed (or rate) at which the crest of a wave moves through a medium. Velocity can be calculated by using the formula: V = lf Where v is velocity of propagation and is expressed in feet (meters) per second, l is the wavelength in feet (meters), and f is the frequency in hertz. REFLECTION occurs when a wave strikes an object and bounces back (toward the source). The wave that moves from the source to the object is called the INCIDENT WAVE, and the wave that moves away from the object is called the REFLECTED WAVE. The LAW OF REFLECTION states: The angle of incidence is equal to the angle of reflection. REFRACTION occurs when a wave traveling through two different mediums passes through the BOUNDARY of the mediums and bends toward or away from the NORMAL. DIFFRACTION can account for the ability of the AM radio waves (due to their low frequency) to travel over a mountain, while FM and TV signals (due to their higher frequencies) are blocked. The DOPPLER EFFECT is the apparent change in frequency of a source as it moves toward or away from a detector. It can affect the operation of equipment used to detect and measure wave energy. SOUND can be audible to the human ear or it can be outside the hearing range. NOISE AND TONES are the two general groups that broadly classify ALL sounds. PITCH, INTENSITY, AND QUALITY are the three basic characteristics of sound. Pitch describes the frequency of sound. Intensity describes how much energy is transmitted. Quality enables us to distinguish one sound from another. The DENSITY of a MEDIUM, TEMPERATURE, and ATMOSPHERIC PRESSURE affect the velocity of sound. If temperature, density, or pressure increases, the velocity of sound increases and vice versa. ACOUSTICS is the science of sound and relates to the sense of hearing. ECHO is an example of reflection. Sound echoes are used in sonar and depthfinders to determine or measure the range of an object or the depth of the ocean bottom. REVERBERATION is the multiple reflections of sound waves. The prolonged roar of thunder is caused by reverberations. With underwater sound equipment, reverberations of nearby objects may interfere with returning echoes from actual targets. INTERFERENCE occurs when two waves move simultaneously through a medium. They can interfere constructively, destructively, or produce a resultant of zero. RESONANCE occurs when an objects vibrates (or resonates) at its natural frequency. When different frequencies are produced inside a cavity, the sound from the cavity sounds louder at its resonant frequency than at all other frequencies. NOISE is any disturbance that distracts from or distorts the quality of sound. A PHOTON is the smallest unit of radiant energy that makes up light waves and radio waves. ANGSTROM units are used for measuring the wavelength of light. One angstrom = 1055-10 m. The VISIBLE SPECTRUM contains all the colors between infrared and ultraviolet. INFRA-RED and ULTRA-VIOLET are invisible to the human eye. The PRIMARY COLORS of light are red, green, and blue. These primaries can be mixed to make any color between red and violet. If the three colors are mixed equally, they produce white light. The COMPLEMENTARY COLORS of light are magenta, yellow, and cyan. They are produced by mixing any two of the primary colors together in overlapping beams. The SPEED OF LIGHT in empty space is considered to be 186,000 miles per second (or 300,000,000 meters per second). This speed varies in different mediums, but the constant of 186,000 miles per second is always used as the speed of light. The ELECTROMAGNETIC SPECTRUM is the complete range of electromagnetic frequencies from 3 kHz to beyond 300,000 THz. Audio frequencies (15 Hz-20 kHz) are not electromagnetic energy and are not included in the electromagnetic spectrum. The ELECTROMAGNETIC FIELD consists of an ELECTRIC FIELD and a MAGNETIC FIELD. These fields are responsible for the transmission and reception of electromagnetic energy through free space.